We present the first rest-frame optical size-luminosity relation of galaxies at z > 7, using the NIRCam imaging data obtained by the GLASS James Webb Space Telescope Early Release Science (GLASS-JWST-ERS) program, providing the deepest extragalactic data of the ERS campaign. Our sample consists of 19 photometrically selected bright galaxies with m (F444W) 7. Given the limited sample size and magnitude range, we first fix the slope to that observed for larger samples in rest-frame UV using Hubble Space Telescope samples. The median size r (0) at the reference luminosity M = -21 decreases slightly from rest-frame optical (600 +/- 80 pc) to UV (450 +/- 130 pc). We then refit the size-luminosity relation allowing the slope to vary. The slope is consistent with beta similar to 0.2 for all bands except F150W, where we find a marginally steeper slope of beta = 0.53 +/- 0.15. The steep UV slope is mainly driven by the smallest and faintest galaxies. If confirmed by larger samples, it implies that the UV size-luminosity relation breaks toward the faint end, as suggested by lensing studies.